1. Field of the Invention
The present invention relates to a multilayer film reflector and a manufacturing method thereof, and particularly to a multilayer film reflector having good reflection characteristics in a soft X-ray region of a wavelength of about 1 to 30 nm, and the manufacturing method thereof. The multilayer film reflector in accordance with the present invention is used in various X-ray optical systems. For instance, it can be used in X-ray lithography, an X-ray telescope and an exposure device.
2. Description of the Related Art
In general, a complex index of refraction for a material is expressed by the equation n =1−δ−ik (wherein n is complex index of refraction; δ and k are real numbers; and k represents absorption of X-ray). The values of δ and k in the equation are much smaller than 1 in a soft X-ray region (wavelength: 1 to 30 nm), so that the complex index of refraction becomes a value extremely close to 1. That is, a light hardly refracts in the soft X-ray region sohardly such that a dioptric system utilizing refraction cannot be theoretically used.
For this reason, a catoptric system (a reflector) utilizing reflection needs to be used in the soft X-ray region. However, when a reflector having a single layer film formed therein is used as the catoptric system, if an incidence angle of a soft X-ray incident on the reflector is small, the reflectance of the reflector becomes extremely low. This is because a marginal angle at which a soft X-ray incident on the reflector is totally reflected (critical angle: θc) is small, such that, for example, the critical angle is 70° or more for a soft X-ray having a wavelength of 10 nm. Specifically, when the reflector is used for an incidence angle of 70° or less, the reflectance for the soft X-rays is so low that the reflector cannot be used.
For this reason, hitherto, for a reflector for the soft X-ray, a multilayer film reflector having two different material layers alternately stacked on each other has been used. In the multilayer film reflector, a layer (high refractive index layer) of a substance having a relatively small difference between a refractive index for a wavelength of a soft X-ray used and a refractive index in vacuum (=1), and a layer (low refractive index layer) of a substance having a relatively large difference between the above refractive indices are alternately stacked on each other in a plurality of layers, with each layer having such an extremely small optical thickness as to be less than a wavelength of an incident soft X-ray. By alternately stacking tens or more of low refractive index layers and high refractive index layers, tens or more of interfaces are formed which are reflecting surfaces for an X-ray of the wavelength used. At this time, if the thickness of each of the low refractive index and the high refractive index layers is designed such that the phases of reflected lights from respective interfaces can coincide, with the use of an optical interference theory, the reflector can develop good reflection characteristics even for a soft X-ray perpendicularly incident on a reflecting surface.
For the combination of a low refractive index layer (refractive index: A) and a high refractive index layer (refractive index: nB), by selecting those two materials which each have as small an absorption coefficient as possible and have as large a difference between nA and nB as possible, a multilayer film reflector having a higher reflectance can be obtained. The combination of two such materials can be selected from several examples. Japanese Patent Application Laid-Open No. H08-262198 discloses a multilayer film reflector which employs Mo for a low refractive index layer and Si a high refractive index layer, and also that an alternate multilayer film of Mo and Si is a substance pair that shows the highest reflectance at a wavelength of 13 nm in the X-ray region. Further, in Japanese Patent Application Laid-Open No. H11-258396, a multilayer film reflector is described which employs Mo for a low refractive index layer and Be for a high refractive index layer. Moreover, in Japanese Patent Application Laid-Open No. H06-230194, a multilayer film reflector is described which employs Ni for a low refractive index layer and C60 or C for a high refractive index layer.
FIG. 4 is a sectional view of a conventional multilayer film reflector 100. In FIG. 4, reference numeral 101 denotes a substrate made of Ni. On the substrate 101, high refractive index layers 104 made of Si (having a refractive index nA of 0.999 in a crystalline state and a thickness of about 5 nm), and low refractive index layers 105 made of Mo (having a refractive index nA of 0.921 in the crystalline state and a thickness of about 4 nm) are stacked. The thickness of each refractive index layer is differently designed according to the order of the formed refractive index layer. The high refractive index layers 104 and the low refractive index layers 105 are alternately stacked into 60 layers to form a multilayer film 102. However, in FIG. 4, the multilayer film 102 is shown as being formed of ten layers for convenience of the description.
In FIG. 4, the thicknesses and number of the refractive index layers are designed so that the reflector can show the highest reflectance when a soft X-ray with a wavelength of 13.5 nm is incident on the reflector at an incidence angle of 20°. The reflection characteristics when an X-ray 106 with a wavelength of 12.5 to 14.5 nm was incident on the multilayer film reflector 100 at an incidence angle of 20° were determined by a simulation and are shown in FIG. 5. It can be seen from FIG. 5 that the multilayer film reflector 100 has such a very high reflectance as 74.4% to an X-ray with a wavelength of 13.5 nm.
The multilayer film reflector 100 of the structure shown in FIG. 4 was actually manufactured, and the reflection characteristics thereof were measured to obtain the result shown as measured values in FIG. 6. In FIG. 6, the theoretical values shown in FIG. 5 are also shown.
It can be seen by comparing the measured values and the theoretical values shown in FIG. 6 to each other that the measured reflectance values are remarkably lower than the theoretical reflectance values obtained from the simulation shown in FIG. 5. Particularly, the measured value to the X-ray with the wavelength of 13.5 nm is as low as 68.5%, which is 5.9% lower than the theoretical value.
The present inventors have studied the reason why the measured reflectance values of the multilayer film reflector 100 shown in FIG. 4 were lowered in comparison with the theoretical values. As a result, the present inventors have found that the refractive index values of Mo constituting the low refractive index films are higher than the theoretical values. Such higher refractive index value of Mo constituting the low refractive index film, resulted in a reduction of the difference in refractive index value between Mo and Si constituting the high refractive index film, whereby the reflectance of the multilayer film reflector 100 was lowered. The details of the study will be now shown below.
At first, Si layers as high refractive index films 105 were in an amorphous state when formed into films on the substrate 101 by sputtering. This is because the interatomic bonding strength of Si is so low that when an Si film is formed by an energy normally used in the sputtering, the crystal lattice is destroyed to form an amorphous structure. In general, the refractive index of an amorphous substance is known to be higher than the refractive index of the substance that is in a crystalline state. The refractive index of Si in a crystalline state is known to be 0.999, so that the refractive index of Si in an amorphous state is considered to be closer to 1 than the refractive index of Si in the crystalline state.
In contrast to this, Mo layers as the low refractive index layers 104 are in an amorphous state as with the Si layers, when formed into films on the high refractive index films 105 by the sputtering. However, because the interatomic bonding strength of Mo is much higher than that of Si, the crystal lattice is gradually restructured immediately after the film formation, so that the crystallization proceeds. Theoretically, the crystallization will proceed to be completed, and the refractive index of the Mo layers should become 0.921 that is the value for a complete crystal. However, the refractive index of the Mo film at the time of completion of the film formation was measured and found to be 0.935. The fact is considered to mean that the formed Mo films are in a quasicrystalline state in which crystallization is not completed.
In addition, as a result of observation of the state of the formed Mo layer through a sectional TEM (transmission electron microscope) image, it was found that the crystallization ratio of the Mo layer in the vicinity of an interface with the Si layer on the substrate 101 side was extremely low, and the crystallization ratio gradually increased toward an interface on the side opposite to the Si layer side. That is, it is considered that the crystallization of the formed Mo layer in the amorphous state is prevented by the Si layer in the amorphous state that functions as a substrate when the Mo layer is formed. It was also found that for this reason, the difference in refractive index between the Mo layer as low refractive index layer and the Si layer as high refractive index layer becomes smaller than a theoretical value, whereby the theoretically derived reflection characteristics could not be obtained from the theory.